This invention relates to a smoke-detecting fire alarm and, more particularly, to an alarm of this type having hysteresis circuitry to ensure an alarm until the alarm condition has ended and a manually actuatable test circuit.
Smoke-sensing fire alarms which sense the change in impedance of an ionization chamber when smoke is introduced thereto are well known. Typically, an open ionization chamber is connected in a voltage divider circuit across a power source and a change in impedance is reflected in a voltage change thereacross. This sensing voltage is monitored by a detection circuit and when it exceeds a preselected alarm level, the detection circuit energizes a suitable alarm circuit. In self-contained, i.e., battery-powered, fire alarms of this type, it is known to provide a battery monitoring circuit which will cause a low battery signal to be generated when the battery has been depleted beneath a level at which successful operation of the alarm circuit is assured.
It is desirable that once the alarm is sounded, it continues to sound until after the condition causing the alarm has terminated. The alarm condition is often represented by an electrical detection signal generated by the circuit exceeding a preselected detection level. One way of ensuring that the alarm remains on until after termination of the alarm condition is to require manual reset or turn-off of the alarm. This, however, may result in unnecessary alarm and depletion of the battery powering the unit due to relative inaccessibility of the alarm for manual reset. On the other hand, if the alarm is designed to automatically turn off at the same preselected detection level of the electrical detection signal which causes the alarm to turn on, electrical or physical transients may cause the alarm to turn off even though the alarm condition has not ended.
It is of utmost importance that the detector be maintained in an operative condition. Accordingly, it is known to provide a manually actuatable test switch and associated circuitry to enable the user to easily test the detector for operability. One type of testing circuitry is shown in U.S. Pat. No. Re. 28,915 in which closure of a test switch causes the application of battery power to an audio horn. Unfortunately, while this tests both the horn and the battery, a positive test result does not necessarily indicate operability of the detection circuit.